cbfs: Add verification for RO CBFS metadata hash

This patch adds the first stage of the new CONFIG_CBFS_VERIFICATION
feature. It's not useful to end-users in this stage so it cannot be
selected in menuconfig (and should not be used other than for
development) yet. With this patch coreboot can verify the metadata hash
of the RO CBFS when it starts booting, but it does not verify individual
files yet. Likewise, verifying RW CBFSes with vboot is not yet
supported.

Verification is bootstrapped from a "metadata hash anchor" structure
that is embedded in the bootblock code and marked by a unique magic
number.  This anchor contains both the CBFS metadata hash and a separate
hash for the FMAP which is required to find the primary CBFS. Both are
verified on first use in the bootblock (and halt the system on failure).

The CONFIG_TOCTOU_SAFETY option is also added for illustrative purposes
to show some paths that need to be different when full protection
against TOCTOU (time-of-check vs. time-of-use) attacks is desired. For
normal verification it is sufficient to check the FMAP and the CBFS
metadata hash only once in the bootblock -- for TOCTOU verification we
do the same, but we need to be extra careful that we do not re-read the
FMAP or any CBFS metadata in later stages. This is mostly achieved by
depending on the CBFS metadata cache and FMAP cache features, but we
allow for one edge case in case the RW CBFS metadata cache overflows
(which may happen during an RW update and could otherwise no longer be
fixed because mcache size is defined by RO code). This code is added to
demonstrate design intent but won't really matter until RW CBFS
verification can be supported.

Signed-off-by: Julius Werner <jwerner@chromium.org>
Change-Id: I8930434de55eb938b042fdada9aa90218c0b5a34
Reviewed-on: https://review.coreboot.org/c/coreboot/+/41120
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Patrick Georgi <pgeorgi@google.com>

1 files changed, 34 insertions, 0 deletions

diff --git a/src/commonlib/bsd/include/commonlib/bsd/metadata_hash.h b/src/commonlib/bsd/include/commonlib/bsd/metadata_hash.h
new file mode 100644
index 000000000000..d5e54b508edb
--- /dev/null
+++ b/src/commonlib/bsd/include/commonlib/bsd/metadata_hash.h
@@ -0,0 +1,34 @@
+/* SPDX-License-Identifier: BSD-3-Clause OR GPL-2.0-only */
+
+#ifndef _COMMONLIB_BSD_METADATA_HASH_H_
+#define _COMMONLIB_BSD_METADATA_HASH_H_
+
+#include <stdint.h>
+#include <vb2_sha.h>
+
+/* This structure is embedded somewhere in the (uncompressed) bootblock. */
+struct metadata_hash_anchor {
+	uint8_t magic[8];
+	struct vb2_hash cbfs_hash;
+	/* NOTE: This is just reserving space. sizeof(struct vb2_hash) may change between
+	   configurations/versions and cannot be relied upon, so the FMAP hash must be placed
+	   right after the actual data for the particular CBFS hash algorithm used ends. */
+	uint8_t reserved_space_for_fmap_hash[VB2_MAX_DIGEST_SIZE];
+} __packed;
+
+/* Always use this function to figure out the actual location of the FMAP hash. It always uses
+   the same algorithm as the CBFS hash. */
+static inline uint8_t *metadata_hash_anchor_fmap_hash(struct metadata_hash_anchor *anchor)
+{
+	return anchor->cbfs_hash.raw + vb2_digest_size(anchor->cbfs_hash.algo);
+}
+
+/*
+ * Do not use this constant anywhere else in coreboot code to ensure the bit pattern really only
+ * appears once in the CBFS image. The only coreboot file allowed to use this is
+ * src/lib/metadata_anchor.c to define the actual anchor data structure. It is defined here so
+ * that it can be shared with cbfstool (which may use it freely).
+ */
+#define DO_NOT_USE_METADATA_HASH_ANCHOR_MAGIC_DO_NOT_USE	"\xadMdtHsh\x15"
+
+#endif /* _COMMONLIB_BSD_MASTER_HASH_H_ */